The invention relates to a sprinkler having a cage for generating fire-fighting foam from a fire-fighting fluid (for example water) containing foaming agent for fighting fires.
Fire-fighting foam is composed substantially of air, water, and a foaming agent containing surfactant. In terms of the foaming agent containing surfactant, the fire-fighting foam in foams that contain fluorine surfactant (′AFFF′ or ‘A3F’ foam fire-fighting agents) and fire-fighting foam that is free from fluorine surfactant can be differentiated, the different modes of action of which will be briefly explained hereunder.
In the use of AFFF foaming agents, the fluorine surfactants by virtue of the chemical structure thereof that differs from that of conventional hydrocarbon surfactants cause a vapour-tight aqueous liquid film of typically 10-30 μm in thickness between the burning surface and the foam. This water film that is caused by fluorine surfactant acts as a barrier to the exit of vapours of burnt material and suppresses any reigniting. By virtue of these properties, AFFF foaming agents are also referred to as ‘film-forming foams’, this being reflected in the acronym thereof of ‘AFFF’ or ‘A3F’ for ‘Aqueous Film Forming Foam’. A further substantial advantage that is linked to the film formation lies in that the foam layer that is bearing on the film does not have to display any outstanding barrier function, that is to say that the fire-fighting foam needs to have only a minor expansion ratio or none at all. In practice, effective fire-fighting using AFFF foaming agents with an expansion ratio of less than 4, typically between 1.5 and 3.0, is therefore adequate, since the major fire-fighting effect is caused by the gas-tight liquid film on the burning surface that is caused by fluorine surfactant. These low expansion ratios mentioned above are achieved by using conventional sprinklers that are conceived for operating with water or AFFF fire-fighting agents. However, these conventional sprinklers by virtue of the construction mode thereof cannot achieve any higher expansion ratio. To this end, one must resort to special sprinklers that are technically complex, for example in the construction mode of a heavy-foam sprinkler such as disclosed in DE 195 39 991 C1, said special sprinklers being approximately 40% to 70% more expensive and are significantly more complex in terms of the construction than the conventional sprinklers.
Heavy-foam sprinklers of this type are installed in particular where AFFF fire-fighting foams by virtue of the damaging properties of the fluorine surfactant component (persistent, bio-accumulative, toxic) must not be used, or where conventional sprinklers that previously have been operated with AFFF fire-fighting foam have to be converted to heavy-foam sprinklers by virtue of the ban on fluorine surfactants. To this end, please refer to the application restrictions of guideline 2006/122/EG. Since fire-fighting foam that is free from fluorine surfactant lacks the film barrier on account of the absence of the fluorine surfactants, the necessary barrier function is thus assigned to substantially only the foam layer. However, the barrier function is only effective at an expansion ratio of 4 and above, at which point said foam is usually referred to as heavy foam, requiring the use of special heavy-foam sprinklers as mentioned above.